Metals - Orthopaedic Biomaterials in Research and Practice - page 174

Composition , fabrication , and heat treatment all affect mechanical
and, to a degree, chemical properties of alloys and should be fully
described in materials selection processes.

Finishing Many steps are required to produce the ideal structures desired for sat-

isfactory performance in biomedical applications. In addition to bulk
processing, there is usually a requirement to adjust or finish the sur-
face of a part. The processes used may result in mechanical or chemical
modification or both.
The following are the principal surface-finishing techniques used
in the manufacture of orthopaedic devices, in alphabetical order. More
than one may be used on any one component, and the order may be
varied. Finishing is still not an exact science, and commercial process
details tend to be guarded.

Anodization. Formation of a strong oxide surface film on aluminum-
base and titanium-base alloys by placing the parts in a suitable con-
ductive (electrolyte) bath and rendering them anodic (in this case,
positively charged) with an external power supply. This essentially
converts the surface of the part from a metal to a ceramic and
improves corrosion and wear resistance very significantly.
Carburization. Deliberate reaction of carbon with steel surfaces pro-
duces local carbide precipitation and very great hardness, although
corrosion resistance may be reduced. This produces a very hard,
tough surface layer over a ductile base, a perfect combination for
a cutting edge. This process was one of the secrets in making the
famous Damascus steel and is still widely used in the manufacture
of scalpel blades, osteotomes, saws, and so on.
Electroplating. Metals may be deposited in a surface film from a suit-
able solution by rendering the part cathodic (in this case, nega-
tively charged). This is frequently done commercially to improve
corrosion resistance, as in the electroplating of chromium onto
automobile bumpers, which are made of non-stainless steel alloys.
This is rarely used in orthopaedic devices for a number of reasons
(see Chapter 10), but electroplated gold coatings are seen on the
handles of some surgical instruments.
Grinding. Physically removing the surface layers by abrasion with
aluminum oxide or silicon carbide on papers or in bonded grind-
ing tools. This may be used as a primary forming process for
hard-to-machine alloys such as the cobalt-base ones or merely to
remove surface impurities, as after investment casting or welding.
Nitriding. Steels and titanium-base alloys may have a portion of their
surface iron or titanium atoms, respectively, converted to nitrides
by reaction with gaseous ammonia or molten potassium cyanate,
both at elevated temperatures. This greatly hardens the surface and,
in the case of titanium-base alloys, imparts a pleasant golden color.
Passivation. All orthopaedic alloys may be passivated by acid treat-
ment, which converts surface elements, particularly chromium